The present invention relates to reclosable packaging and, in particular to a method for treating zipper used in the production of such packaging to remove camber from the zipper.
Reclosable packaging is becoming increasingly more popular as primary packaging for food stuffs and other commodities. One reason for the growing popularity of such packaging is the development of form, fill and seal (FFS) equipment which lends itself to rapid and mass production of packages with advanced features, such as reclosability combined with hermetic sealing and tamper evidence. A common form of FFS equipment feeds a length of zipper from a spool, applies the zipper length transversely across a bag film and then feeds the film and applied zipper over a forming collar and about a fill tube to form and ultimately fill the bag.
A problem that has been encountered in such operations is that the zipper when spooled tends to develop curvature or camber. This is caused by the difference in diameter between the interlocking profiles when wound around the core of a spool. The lower profile or the profile closest to the core has a smaller diameter than the engaged profile above it. This difference is more acute at the locations closests to the core. Even after the zipper is attached to the film, the camber remains, particularly when the film is relatively flexible. This causes alignment and other problems in the production process and could lead to an unsightly and sometimes unworkable package.
In view of the above, a principal object of the present invention is to provide a method of treating zipper to remove such camber in the zipper.
A further object is to provide such a method that can be practiced at any point in the production of the reclosable packaging but preferably after the formed zipper is un-spooled but before it is sealed in place.
Still another object is to provide such a process that employs relatively simple equipment to remove camber that results from the zipper spooling.
Accordingly, in accordance with the present invention, camber is removed from a spool wound zipper, by stretching the zipper longitudinally around a curved surface so the interlocking elements are stretched relative to each other. Generally the zipper interlocking elements are stretched against a surface that is in a plane of about 90 degrees to the direction in which the elements interlock. In other instances to further help remove the camber, the zipper may be driven around the curved surface in a direction opposite to that in which it had been wound on the spool. To this end, a set of drive rolls drives a zipper that was wound on a spool past a curved surface, with the zipper fitting into a groove in the curved surface shaped to accommodate it. A set of guide rollers maintain pressure on the zipper and hold it in position. Alternately the zipper is fed to an idler wheel from a set of first drive wheels and fed from the idler wheel by a set of second drive wheels. The second drive wheels are driven faster than the first drive wheels to effect the stretching. The idler wheel can be spring loaded with a calibrated spring to add additional stretch to the zipper. Alternately only the second set of wheels are used to drive the zipper, with the spring loaded idler wheel providing the required stretch. The rim of the idler wheel has a center channel in which the profiles"" interlocking members ride and the center channel has sides that will accommodate the zipper shape.